The present invention relates to an input buffer circuit for receiving digital data signals from a transmission line according to the preamble of claim 1. An input buffer circuit of this kind is known from EP-A-0 504 060.
Digital circuits and technology are the key for systems with an ever increasing performance and functionality. This goes together with a continuously increasing scale of integration and an increase in the complexity of digital systems. Different sections of such systems communicate via impedance matched transmission lines which constitute an inexpensive and convenient means for carrying data rates in the order of several hundred Mbit/s over short and medium distances within a digital system.
In complex systems, power consumption is a major point of concern. The more complex and compact the system, the more important it is that each system component uses as little operating power as possible. Differential signalling with low signalling voltages is well suited to meet the requirement of low power transmission and, due to the symmetric nature, also the requirement of noise immunity.
In order to transmit data via a symmetrical transmission line, a line driver circuit is required at the transmitting side of the transmission line in order to provide low impedance data signals. The amplitude of these data signals is low, in order to keep the power transmitted over the transmission line with a given characteristic impedance, reasonably small. At the receiver side of the transmission line an input buffer circuit is provided, for receiving the digital data signals and for outputting a corresponding digital signal to subsequent sections.
In large systems, e.g. if standardized interfaces between system subsections are provided, the number of available channels is sometimes higher than the number of actually needed channels. Regardless, whether an available channel is used or not, for each available channel an input buffer circuit is provided which will consume operating power. Of course, each input buffer could be individually connected to the power supply or disconnected from the power supply. This, however, would require special elements like jumpers or switches for allowing individual settings. Additional space would be required and the overall system reliability would be lower, and additional costs would arise in the process of manufacture.
It is, therefore, an object of the present invention to provide an input buffer circuit for receiving digital data signals from a transmission line, wherein the circuit is very power efficient even if it is not currently being used in a specific system configuration.